1. Field of the Invention
The present invention relates to lamp ballasts and, more particularly, to lamp ballasts which are suitable for exciting electrodeless, high-intensity-discharge lamps.
2. Background Information
In the lighting field, it has long been desired to provide long lifetime, low maintenance, low life cycle cost lighting systems. One of the lamp life limiting factors which increases the life cycle cost of common incandescent lights and fluorescent lights is their dependence on the filament in the case of the incandescent light, and on electrodes in the case of the fluorescent light for establishing and maintaining the light output. While the periodic replacement of such lamps in a domestic setting is an inconvenience, the only direct cost of replacing such lamps is normally the cost of the replacement bulb. In contrast in commercial settings, the labor cost for replacing a spent bulb often approaches or exceeds the cost of the replacement bulb, even in easily accessible fixtures. In hard to reach locations, such as high ceilings, light stanchions in parking lots and other relatively inaccessible locations where mechanical lifting equipment is required in order to reach the fixture to replace the spent bulb, the cost of replacing a bulb normally vastly exceeds the cost of the bulb itself.
Another major concern in commercial lighting is power efficiency because many commercial establishments contain hundreds, thousands or even tens of thousands of lamps with the result that even a small change in the power efficiency of a lighting system can result in substantial power and cost savings. In addition, if a particularly high efficiency lighting system generates more light than corresponding lower efficiency systems, the number of lamps required to light large areas may be reduced, thereby saving the initial capital cost of the omitted fixtures, along with their wiring and installation costs in addition to their ongoing operating costs.
An overall power efficiency measure of a lamp system is the quantity of light output (which may be measured in lumens) which is provided per watt of electrical power input. The power loss in a lighting system may be divided into two separate components. The first power loss component is the portion of the power consumed by the lamp fixture as a whole which is not actually delivered to the light production medium, whether that be the filament of an incandescent lamp, the arc of a fluorescent lamp or the ionizable, arc-producing gas of a gas discharge lamp. Power losses which help to prevent 100% efficiency in the transmission of electrical power to the light producing medium include such things as electrical resistance within the leads leading to a filament, the power loss within the ballast of a fluorescent lamp and so forth. The second power loss component is the portion of the electrical power actually delivered to the light producing medium which does not emerge from the lamp in the form of usable (visible spectrum) light. One of the primary contributors to this form of loss in incandescent and gas discharge lamps is heat generation within the lamp itself.
One adverse effect of power losses, both within the electrical portion of the system and within the light producing portion of the system, is the accompanying generation of heat which must be carried away from the lamp and its fixture in a safe, reliable manner. This is particularly a problem with gas discharge lamps which operate at a high temperatures. Further, the heat generated by a gas discharge lamp can have adverse effects on semiconductor devices used in the lamp ballast which produces the high frequency signal needed to sustain the arc discharge within the lamp.
In the past, lamps, including their fixtures and their bulbs, have generally been relatively large with the result that the size of the ballast which excites a fluorescent or other discharge lamp has not been a major concern in lamp designs because the physical dimensions of the ballast were normally substantially smaller than the fixture dimensions which were required merely by the size of the bulb or because the ballast was easily incorporated into the base of a desk or floor lamp.
The electrodeless, high-intensity-discharge (H.I.D.) lamp was developed in an attempt to increase bulb life and light output to thereby reduce life cycle costs for lighting systems. Lamps of this general type are disclosed in U.S. Pat. Nos. 3,500,118 and 4,180,763, both issued to J. M. Anderson and in U.S. patent application Ser. No. 134,498, filed Dec. 17, 1987, by Sayed-Amr El-Hamamsy et al. Such lamps have the potential to operate almost indefinitely with their original bulb, since the bulb contains no electrodes which can deteriorate, relies on an arc discharge within a gas contained within its envelope for its light emission and uses an externally applied, localized, radio frequency signal to create and sustain that arc discharge. Such lamps may be generally toroidal in the sense of comprising a doughnut-like continuous, hollow, annular tube which contains the gas in which their arc discharge forms. Such generally toroidal lamps may be circular, elongated or other desired shapes. Other electrodeless, high-intensity-discharge lamps may employ a generally spherical, ellipsoidal or elongate hollow envelop disposed within an excitation coil to contain their arc discharge gas.
Electrodeless, high-intensity-discharge lamps require a high frequency ballast to convert ac power line frequencies to the radio frequencies which are necessary to induce an arc discharge in the gaseous medium disposed within the lamp envelope. Because the system power is large (300 W or more) a lot of power is dissipated even at efficiencies of up to 90%. Most of this power is dissipated in the switching devices, which therefore require efficient heat sinking. Proper heat sinking of the switching devices protects the junctions of these devices from excessive temperature rises. Excessive temperature rises have two main deleterious effects. The first is that as the junction temperature rises, so does the on resistance of the device. This increased on resistance increases the power losses. The second deleterious effect is that if the devices run too hot, their life is significantly reduced.
In order to minimize life cycle costs for lighting systems, there is a need for high efficiency lamp ballasts for exciting discharge lamps of all kinds. This need is especially significant with electrodeless, high-intensity-discharge lamps whose enormous bulb life makes power costs a more significant component of the life cycle costs than is the case with lamps which require frequent, or at least more frequent bulb replacement.
Electrodeless, high-intensity-discharge lamps of the type having a generally ellipsoidal gas containing envelope may have a lamp envelope which is substantially smaller than typical prior art ballast systems. With such small lamp envelopes, it becomes desirable to place the ballast in close proximity to the bulb's envelope in order to provide a compact fixture. More important from an efficiency point of view is the need to minimize the length of the leads from the ballast to the drive coil, since the relatively large RF currents flowing in these leads can cause substantial resistive losses. Where the ballast of such a lamp includes semiconductor devices or other particularly heat sensitive components, a desire to place the ballast close to the lamp for compactness reasons must be weighed against the adverse effect of exposing the ballast, and particularly its semiconductor devices, to the higher temperatures which result from placing the ballast close to the bulb.
Accordingly, there is a need for compact, high efficiency ballast systems for excitation of electrodeless, high-intensity-discharge lamps whose efficiency and useful life are not adversely affected by being disposed in close proximity to a high temperature gas discharge lamp envelope.